The rotor of conventional electric motors is built using a stack of laminated magnetic steel discs. The lamination prevents the flow of currents between the discs that can interfere with the efficiency of the rotor. The discs have numerous teeth radiating out from a circular portion, with spaces in between the teeth. The discs are stacked with the teeth above one another, forming a cylinder with deep grooves formed by the spaces between the teeth of the discs.
Conductors such as aluminum or copper are added to the grooves, either by inserting aluminum or copper bars into the grooves, or casting molten aluminum into the grooves. The conductors are longer than, and thus stick out from the ends of, the stacked discs. At either end, the conductors are electrically connected to one another: in the case of casting, the mold allows the connection to occur at the ends of the conductors, and in the case of bars being added, the ends of the bars can be electrically connected by inserting slugs in the spaces between the ends of the bars, and brazing the slugs to the bars using a conventional brazing technique. One such brazing technique involves melting a rod of brazing material into heated joints between the bars and the slugs, causing an electrical and mechanical connection between them.
A motor can be more efficient if the resistance between the conductors is reduced. However, the heavier the motor is, the more inertial resistance that motor will have, and so a heavier motor may not be as efficient as a lighter motor with the same dimensions. Because aluminum is lighter than copper, it has a lower inertial resistance, but because aluminum is not as good a conductor as copper, motors built from aluminum can have a higher electrical resistance than they could if they were built using an identical geometry of copper, at least partially offsetting the inertial efficiency. Offsetting the higher electrical resistance of aluminum is the fact that the interconnections can be cast with the conductors, providing only minimal resistance between the conductors. Conventional brazing techniques used for copper may not produce an electrical connection with resistance that is as low.
Cast copper could have the benefits of the lower resistance of copper, while achieving the higher conductivity of casting, but casting copper requires temperatures that could damage the laminated discs, and thus, cast copper conductors and connections between the conductors have not been frequently used in this manner. The brazing technique of electrically connecting the copper bars has a higher resistance than would result from casting, because gaps remain between the surfaces of the slugs and the bars after brazing. However, because casting copper is impractical, brazing the copper bars together to form the end rings of the rotor continues to be used.
One way around these problems with copper bars is to connect them with a single cap piece of silver-plated copper that is cast in a manner that has fins where the slugs would have been positioned, but is cast into a single piece, with connections at the ends where cast connections could have been employed had the conductors been cast instead of being made of multiple bars. The fins can be plated with braising material. The cap piece is slipped over the ends of the bars and then induction braised to the bars. However, because the fins of the cap piece must be slipped in between the bars all at the same time, the tolerances of the fins occupying the spaces between the bars can not be significantly tight, increasing the resistance of the electrical connection. Even after brazing, gaps remain between the fins and the bars because of the wide tolerances required to insert the fins in the spaces between the bars. Although the resistance between the bars of such a construction can be lower than braising, it can still be significant. Furthermore, the cast connections at the ends can add enough weight to the rotor to increase its inertial resistance, reducing the efficiency of the rotor.
What is needed is a system and method that can electrically connect the ends of copper bars added to a rotor that can lower the resistance of the electrical connection between them without the added rotational inertia of a cap piece.